Influenza is a major cause of morbidity and mortality nearly every year;when pandemics occur, the impact is even more severe. A pandemic virus may result from reassortment of human influenza viruses with animal influenza viruses. Such reassortment may occur more readily in tropical climates where human and animal viruses frequently co-circulate throughout the year. Few data are available to document the transmission and burden of influenza in tropical developing countries. Without a better understanding of influenza epidemiology in such situations, it is difficult for national and global public health officials to develop influenza-control strategies for either seasonal or pandemic influenza. Effective inactivated influenza vaccines have been available for decades, but data are lacking on their effectiveness for both protecting recipients in tropical developing countries and also their ability to reduce transmission in such populations. A number of reports from developed countries indicate that influenza vaccine, when given to a limited number of persons most responsible for transmission, usually children, has the potential to interrupt transmission and reduce the overall influenza burden in a community. The potential of influenza vaccine to provide indirect protection to unvaccinated individuals suggests that targeted use, which may be feasible in developing-country settings, might reduce influenza infection in the community beyond those who were vaccinated. In a pandemic situation, these efforts could slow the spread of pandemic virus and provide sufficient time for public health officials to introduce interventions to contain or blunt the effects of a pandemic. Thus, to evaluate the effectiveness of trivalent, inactivated influenza vaccine in a tropical developing country, we propose to conduct an observer- blind, cluster-randomized, controlled, parallel-group trial of a new adjuvanted influenza vaccine in Senegal. The trial will take place over a three-year period and will target children 6 months to 10 years of age for vaccination. Not only will effectiveness be estimated for child vaccine recipients, but indirect effectiveness will also be measured for the rest of the community. Throughout the course of the study, both sentinel and active surveillance will be conducted to document the occurrence of febrile acute respiratory illness among participants and in the community. The primary outcome will be prevention of symptomatic laboratory- confirmed influenza meeting a case definition. Surveillance will also allow us to describe the epidemiology of influenza in a tropical developing country by defining the rates of laboratory-confirmed influenza and by describing the clinical characteristics of influenza in the population.